(1) Field of the Invention
This invention relates to a process for the preparation of a nuclear halide of a monoalkylbenzene. More particularly, the present invention relates to a process for preparing a p-halogeno-monoalkylbenzene at a high selectivity by nuclear halogenation of a monoalkylbenzene in the presence of a specific catalyst.
(2) Description of the Prior Art
Nuclear halides of monoalkylbenzenes are valuable as starting materials for the synthesis of medicines, agricultural chemicals, and other various organic compounds of these, p-chloro-monoalkylbenzenes, for example, p-chlorotoluene, are in especially great demand.
As a conventional process for the preparation of nuclear halides of monoalkylbenzenes, there is known a process in which a monoalkylbenzene is subjected to nuclear halogenation by using a Lewis acid such as antimony chloride, ferric chloride, or aluminum chloride as a catalyst and a chlorine gas as a chlorinating agent. In this process, an o-chloro-monoalkylbenzene is formed as a main product and an m-chloro-monoalkylbenzene and polychlorinated substitution products are formed as by-products. It is impossible, however, to prepare the p-chloro-monoalkylbenzene in a yield higher than 40%.
Various catalysts have therefore been developed so as to produce p-chloro-monoalkylbenzenes in high yields. For example, in a process using a Lewis acid and sulfur or selenium as the catalyst, a p-chloro-monoalkylbenzene is obtained in a yield of 45% to 52%. In a process using a Lewis acid and thianthrene as the catalyst, a p-chloro-monoalkylbenzene is obtained in a yield of 55% to 60% (see U.S. Pat. No. 4,031,147 and British Pat. No. 7,605,039). Furthermore, in a process using a Lewis acid and phenoxthine as the catalyst, a p-chloro-monoalkylbenzene is obtained in a yield of 52% to 60% (see European Pat. No. 63384).
As another process for preparing a nuclear halide of a monoalkylbenzene, there is known a process in which silica gel is used as the catalyst and sulfuryl chloride is used as the halogenating agent. In this process, a p-chloro-monoalkylbenzene is obtained in a yield of 50% [see Journal of Synthetic Organic Chemistry, 37, page 690 (1979) (Japan)].
However, all of these known processes are still insufficient in the selectivity to p-chloro-monoalkylbenzenes.